The endogenous existence of carbon monoxide (CO)-activated protective signalling pathways, including vasodilatory, antiapoptotic, antithrombotic, anti-inflammatory and immunomodulatory, has previously been established. Delivery of CO gas at low concentrations protects organs from ischaemic injury by decreasing cell death and most significantly inducing a preconditioning response. Carbon monoxide has been found to bind to hemoproteins, such as, myoglobin, soluble guanylate cyclase (sGC), inducible nitric oxide synthase, cytochrome p-450, cytochrome-c oxidase, NADPH oxidase and the parent enzyme heme oxygenase. This CO interaction can modulate the activity or level of expression of these key protein targets to produce a plethora of downstream signalling events. The anti-inflammatory effects of CO are mediated by p38 MAPK signalling and inhibition of the potent pro-inflammatory damage recognition molecule HMBG1 results in a reduction of pro-inflammatory cytokines, such as TNF-α, and increase in the expression of the anti-inflammatory interleukin-10.
The protective and physiological effects of low levels of CO have been studied in a number of both animal and clinical models with the literature extensively reviewed1. The results of previous studies have been so compelling that the FDA has granted approval for the application of low dose CO gas in a range of clinical trials such as CO gas delivery by inhalation in heart valve replacement surgery2 and in renal transplant procedure recipients3. Inhalation of CO (100-125 ppm) in chronic obstructive pulmonary disease patients, reduced sputum neutrophils and improved bronchial responsiveness. In renal transplant patients low dose CO gas (2 or 3 mg/kg) during surgery has been demonstrated to improve post-transplant kidney function (reduced serum creatinine and increased glomular filtration rate; GFR). However, the difficulty of controlling gas delivery in a clinical setting, combined with the hazardous consequences of any gas leak, have been acknowledged as significant impediments in the use of CO in gaseous form. Metal-based CO-releasing molecules have been developed as an alternative CO delivery system with the advantage of greater ease of administration and control, potentially enabling a safer, tightly controlled method of low dose CO delivery. These molecules have been extensively studied and their beneficial effects demonstrated in transplantation and disease models (e.g. Caumartin, 2011)4. However, currently available molecules contain metal cores, which have potential to induce toxic effects.
In relation to organ removal and transplantation, there has been little improvement in clinical outcomes in non-heart beating transplantation since the early 1980's yet cadaveric organ donation provides an under-utilised source of additional organs with which to boost the donor pool. All transplant organs are subject to rapid deterioration during storage. This attrition adds further pressure to the considerable imbalance between the numbers of available donors compared to patients requiring transplant and procedures able to be undertaken. While dialysis is now considered as a “bridge to transplant”, the average life expectancy of a kidney dialysis patient is reduced fourfold compared to healthy age-matched individuals, with only 35% of dialysis patients expected to survive beyond 5 years. Cadaveric organs traditionally have worse outcomes than grafts obtained from live or brain stem dead donors on life support. However, CO has been shown to improve organ function and survival, even in cross specie animal transplant studies. Studies have affirmed the feasibility and desirability of developing CO releasing molecules to harness the therapeutic effects of CO. However, several studies have highlighted the toxic effects of current metal based CO releasing molecules.
Consequently, there is a clear need to develop novel, alternative compounds and agents capable of releasing carbon monoxide for the treatment or conditioning of subjects or cells, tissues or organs obtained from subjects. It would also be desirable to develop carbon monoxide releasing agents which have properties for protecting transplanted cells, tissues or organs, provide low toxicity and are suitable for use in perfusion formulations.